1. Field of the Invention
The present invention relates to an exhaust gas purification system for an internal combustion engine which can purify nitrogen oxides (NOx), hydrocarbons (HC), and carbon monoxide (CO) included in exhaust gas from an internal combustion engine both when the engine is being warmed-up and after the engine has been warmed-up.
2. Description of the Prior Art
Combustion at lean air-fuel ratios (lean burn) is effective to improve fuel economy of automobile internal combustion engines. However, in the lean burn engine, NOx reduction by a three-way catalyst cannot be expected, and therefore, an alternative means for reducing NOx needs to be developed.
As a catalyst capable of reducing NOx even under an oxidizing exhaust gas condition of the lean burn engines, Japanese Patent Publication HEI 1-130735 discloses a zeolite catalyst carrying transition metals which can reduce NOx in the presence of hydrocarbons (hereinafter, a lean NOx catalyst).
Also, Japanese Patent Publication HEI 1-139145 discloses a catalyst arrangement wherein an oxidizing catalyst or a three-way catalyst is arranged downstream of a lean NOx catalyst located in an exhaust conduit of an internal combustion engine so that HC and CO emissions which have passed through the lean NOx catalyst without being purified can be purified by the oxidizing or three-way catalyst.
However, in the prior art catalyst arrangement, since the oxidizing or three-way catalyst is located relatively far from the engine because of the lean NOx catalyst located between the engine and the oxidizing or three-way catalyst, and since temperature of exhaust gas itself is relatively low due to the lean burn, the oxidizing or three-way catalyst cannot be quickly warmed-up. As a result, the oxidizing or three-way catalyst cannot effectively purify HC and CO emissions during the warming-up of the engine, though a relatively large amount of HC and CO emissions is included in the gas flowing from the engine to the catalyst during the warming-up.